1. Field of the Invention
The present invention relates to a photoresist exposure method. In particular, the invention involves a method which a photosensitive resin (electrodepositional photoresist) coated on the surface of a board for printed wiring and on the inner wall surfaces of through-holes therein by an electrodeposition process is exposed to light through a film.
2. Description of the Related Art
A method of producing a printed-wiring board may be divided into two types, that is, the subtractive method and the additive method. The latter additive method, in which a conductive circuit pattern is formed directly on only a necessary portion of an insulating base by electrolysis plating, needs a high level of technique and process control for maintaining the physical properties of the film formed by electrolysis plating and is therefore not widespread. The former subtractive method, in which a copper-clad laminate is etched to remove the copper therefrom except for the copper pattern defined as the circuit pattern, is presently the leading method for forming a precise circuit pattern.
In the subtractive method, a photosensitive resist film is not hot-laminated on the surface of a copper-clad laminate having been subjected to through-hole plating. Further, and after the resist film is exposed to ultraviolet rays through a negative film, the negative film is removed, and then development is carried out to remove the non-exposed portions of the resist film. Since no conventional photosensitive resist can coat the inner wall surface of the through-holes, there has heretofore been a need for a means for protecting the through-hole plating conductor from the developer and the etching liquid. Examples of such protecting means include the method (tenting method) wherein the copper-clad laminate is directly etched with the through-holes being covered with a dry film or a resist ink, and the method (solder through-hole method) wherein solder is plated over both the conductive circuit pattern and the through-hole plating conductor to form a solder layer which is used as an acid resistant film during the etching process.
However, employment of the polymer electrodeposition method developed recently enables the resist to be coated even on the inner wall surface of the through-holes and hence permits etching to be carried out with the through-holes being covered with the electro-deposit resist after development. According to the polymer electrodeposition, a printed circuit board which has been subjected to through-hole plating is coated by electrodeposition with a photosensitive resin (electrodeposition photoresist) having two functions, that is, electrodepositional and photosensitive functions. Since there is no need for tenting, the polymer electrodeposition has attracted attention as being a method which enables formation of a resist pattern with high resolution.
In the polymer electrodeposition method described above the photoresist, that is uniformly electro-deposited on the surface of the printed circuit board subjected to through-hole plating and on the inner wall surface of the through-holes therein, is exposed to ultraviolet rays through a film by an exposure apparatus. Since there is a large difference in the amount of irradiation energy required between the exposure of the resist for forming a circuit pattern on the surface of the printed circuit board and the exposure of the resist on the inner wall surface of the through-holes, it has heretofore been impossible to use a single exposure apparatus for the two exposure processes. The reasons for this will be explained below in detail. The electro-deposited photoresist is formed with a uniform coating thickness over the board surface and the through-hole inner wall surfaces, but the through-holes vertically extend through the board from the surface thereof and therefore have a depth (the length of the resist layer corresponding to the thickness, or depth, of the through-holes is about 80 times the thickness of the resist coating on the board surface). Further, the through-hole diameter is small, i.e., 0.25 to 0.80 mm, so that light cannot readily enter the through-holes. In addition, the number of through-holes is large, i.e., 5,000 to 13,000 per board. For these reasons, the exposure of the resist on the inner wall surfaces of the through-holes requires a larger amount of irradiation energy than that needed for the exposure of the resist for forming a circuit pattern on the surface. Accordingly, for an adequate amount of irradiation energy applied to the surface of the board, the resist on the inner wall surfaces of the through-holes is left uncured due to a shortage of irradiation needed for the through-holes energy. If the capacity of the light source is increased and the exposure energy is extremely increased in order to sufficiently expose the resist on the through-hole inner wall surface, there will be an adverse effect on the formation of a precise circuit pattern on the surface of the board. More specifically, if the quantity of light is excessively large, the following problems arise: the light undesirably penetrates to the positive portions of the film; expansion and contraction of the film occurs due to the heat from the light; the resist may be destroyed; and the line width of the circuit pattern becomes think or thin, which makes it difficult to form a precise pattern.
Accordingly, it has heretofore been necessary to provide two light sources, that is a light source for exposure of the resist for forming a circuit pattern on the broad surface and a light source for exposure of the resist on the inner wall surfaces of the through-holes, and to carry out exposure with rays of light with a given amount of irradiation energy from each light source disposed at a given distance. Thus, the prior art process is inefficient and inferior in the precision of the resulting circuit pattern.